The present disclosure contemplates instrumentation and procedures for achieving spinal fixation and more particularly to a screw driver for driving a bone screw into a vertebra of a patient.
A typical spinal fixation system 10 as shown in FIG. 1 spans between successive vertebrae V of the spine. An elongated member, such as rod 12, extends along the length of the spine and provides an anchor point for connecting each vertebra to the rod. The rod is typically contoured to approximate the normal curvature of the spine for the particular instrumented spinal segments, which may include lordosis or kyphosis. Anchor devices 15 are provided for connecting the vertebral segments to the elongated member. These anchor devices may include hooks, bolts, screws or other means for engaging a vertebra. For the purposes of the present discussion, the anchor device 15 is a bone screw assembly, such as the screw assembly shown in FIG. 2.
The bone engaging fastener or screw assembly 15 includes a shank 16 that carries threads configured to engage vertebral bone. For instance, the fastener is a multi-axial pedicle screw with a shank that is threaded for engagement within the pedicle of the vertebra. The screw assembly further includes a head 16a by which the screw, and ultimately the vertebra, is fastened to the spinal rod 12. In particular, the head 16a supports a yoke 17 that is generally U-shaped to receive the spinal rod therethrough, as depicted in FIG. 2. The rod 12 may be supported in part by a collar 18 mounted over the head 16a of the bone screw. A cap 19 may carry a set screw 20 that locks the rod within the yoke 17 and thus fastens the rod 12 to the bone screw or the set screw 20 may be threadably attached directly to the yoke 17.
One embodiment of a bone screw assembly 15 is disclosed in co-pending, commonly assigned U.S. application Ser. No. 11/762,898 (the '898 Application), entitled “Multi-Axial Fixation Assembly”, filed on Jun. 14, 2007 and published as No. 2008/0119858, the disclosure of which is incorporated herein by reference in its entirety. For the purposes of the present disclosure, the bone screw 15 may be constructed as disclosed in the '898 Application, although it is understood that other multi-axial bone screw configurations may be implanted using the instruments and procedures disclosed herein. In the multi-axial bone screw assembly 15 the yoke 17 is articulatingly attached to the threaded bone screw 16, and more specifically to the head 16a of the bone screw, so that the yoke 17 can adopt a range of spherical angles relative to the bone screw. Thus, the yoke can articulate relative to the bone screw fastened in the vertebra so that the slot 42 can be aligned to receive the connecting rod 12.
While in the past spinal fixation systems using screws of the '898 Application have been implanted in open procedures involving relatively large incisions through the patient's tissue with significant muscle retraction, more recent procedures have been developed to introduce spinal fixation systems in a minimally invasive or percutaneous manner. With multi-axial pedicle screws being primarily used in these systems there is a need to provide instruments for the surgeons to properly and readily insert such screws into the vertebrae of the spine for suitably receiving and supporting spinal connecting rods.